The present invention relates to a semiconductor laser device, in particular, to a solid-state laser device, and further relates to a solid-state laser device which is oscillated in two wavelengths by a resonator and converts the wavelength in the resonator.
A diode pumped solid-state laser is known, which uses intracavity type SHG mode to convert frequency of a laser beam from a fundamental frequency.
Referring to FIG. 5, description will be given on general features of the diode pumped solid-state laser of one-wavelength oscillation.
In FIG. 5, reference numeral 2 denotes a light emitter, and 3 is an optical resonator. The light emitter 2 comprises an LD light emitter 4 and a condenser lens 5. Further, the optical resonator 3 comprises a laser crystal 8 where a dielectric reflection film 7 is formed, a nonlinear optical medium (NLO) 9, and a concave mirror 12 where a dielectric reflection film 11 is formed. A laser beam is pumped at the optical resonator 3, and the laser beam is outputted by resonation and amplification. As the laser crystal 8, Nd:YVO4 may be used. As the nonlinear optical medium 9, KTP (KTiOPO4; titanyl potassium phosphate) may be used.
Further description will be given below.
A laser light source 1 is used to emit a laser beam with a wavelength of 809 nm, for instance, and the LD light emitter 4, i.e. a semiconductor laser, is used. The LD light emitter 4 has the function as a pumping light generator for generating an excitation light. The laser light source 1 is not limited to the semiconductor laser, and any type of light source means can be adopted so far as it can emit a laser beam.
The laser crystal 8 is used for amplification of light. As the laser crystal 8, Nd:YVO4 with an oscillation line of 1064 nm is used. In addition, YAG (yttrium aluminum garnet) doped with Nd3+ ion or the like is adopted. YAG has oscillation lines of 946 nm, 1064 nm, 1319 nm, etc. Also, Ti (sapphire) with oscillation lines of 700-900 nm can be used.
On the LD light emitter 4 side of surfaces of the laser crystal 8, a first dielectric reflection film 7 is formed. The first dielectric reflection film 7 is highly transmissive to a laser beam from the LD light emitter 4 and is highly reflective to an oscillation wavelength of the laser crystal 8, and it is also highly reflective to SHG (second harmonic generation).
The concave mirror 12 is designed to face to the laser crystal 8. The laser crystal 8 side of surfaces of the concave mirror 12 is fabricated in form of a concaved spherical mirror having an adequate radius and a second dielectric reflection film 11 is formed on it. The second dielectric reflection film 11 is highly reflective to the oscillation wavelength of the laser crystal 8, and it is highly transmissive to SHG (second harmonic generation).
As described above, when the first dielectric reflection film 7 of the laser crystal 8 is composed with the second dielectric reflection film 11 of the concave mirror 12 and the laser beam from the LD light emitter 4 is pumped to the laser crystal 8 via the condenser lens 5, the light is reciprocally projected between the first dielectric reflection film 7 of the laser crystal 8 and the second dielectric reflection film 11. Thus, the light can be confined for longer time, and the light can be resonated and amplified.
The nonlinear optical medium 9 is inserted in the optical resonator, which comprises the first dielectric reflection film 7 of the laser crystal 8 and the concave mirror 12. When an intensive coherent light such as a laser beam enters the nonlinear optical medium 9, a second harmonic wave to double the light frequency is generated. The generation of the second harmonic wave is called “second harmonic generation (SHG)”. As a result, a laser beam with a wavelength of 532 nm is emitted from the laser light source 1.
In the laser light source 1 as described above, the nonlinear optical medium 9 is inserted into the optical resonator, which comprises the first dielectric reflection film 7 of the laser crystal 8 and the concave mirror 12, and it is called an intracavity type SHG. Because conversion output is proportional to square of excited photoelectric power, there is such effect that high light intensity in the optical resonator can be directly utilized.
Further, a type of solid-state laser device is known, by which an entered laser beam of a fundamental frequency is oscillated to two different wavelengths and these are further converted to different frequencies by using sum frequency mixing (SFM) and differential frequency mixing (DFM).
Description will be given on the solid-state laser device as described above referring to FIG. 6. In FIG. 6, the LD light emitter 4 and the condenser lens 5 are omitted.
As seen from the LD light emitter 4, there are arranged a concave mirror 12, a laser crystal 8, a first plane reflection mirror 14, a nonlinear optical medium 9, a second plane reflection mirror 15, and a third plane reflection mirror 16.
The concave mirror 12 is highly transmissive to a wavelength λi (809 nm in the figure), and it is highly reflective to a wavelength λ1 (1342 nm in the figure) and a wavelength λ2 (1064 nm in the figure). The first plane reflection mirror 14 is highly reflective to SFM (wavelength λ3=593 nm in the figure) and is highly transmissive to the wavelengths λ1 and λ2. The second plane reflection mirror 15 is highly transmissive to the wavelengths λ3 and λ2, and it is highly reflective to the wavelength λ1. The third plane reflection mirror 16 is highly transmissive to the wavelength λ3 and is highly reflective to the wavelength λ2.
The excitation light λi entered through the concave mirror 12 excites the laser crystal (Nd:YVO4). Among the natural released light beams, the light beams with the wavelengths λ1 and λ2 are pumped and resonated between the concave mirror 12 and the second plane reflection mirror 15 and between the concave mirror 12 and the third plane reflection mirror 16. The wavelength of λ1 is excited and amplified, and the wavelength of λ2 is excited and amplified. Further, the laser beams with both wavelengths pass through the nonlinear optical medium 9. As a result, sum frequency λ3 of both wavelengths can be obtained, and the laser beam passes through the third plane reflection mirror 16 and is projected.
In case of sum frequency mixing (SFM), there exists a relationship: 1/λ3=1/λ1+1/λ2. By selecting the nonlinear optical medium 9, differential frequency mixing (DFM) can be obtained. In this case, there exists a relationship: 1/λ3=1/λ1−1/λ2 (where λ1<λ2).
In the frequency conversion of the above described solid-state laser device for generating sum frequency mixing (SFM) and differential frequency mixing (DFM), it is advantageous in that wavelength conversion can be achieved with high efficiency by arranging the nonlinear optical medium 9 in the optical resonator.
A conventional type example as described above is written in, for instance, F. chen. and S. W. Tssi: Opt. Lett. 27 (2002), 397.
In the solid-state laser device shown in FIG. 6, sum frequency mixing (SFM) and differential frequency mixing (DFM) are generated, and frequency conversion is performed. It is advantageous in that wavelength conversion can be carried out with high efficiency, while it has the following disadvantages:
The laser beam, which can be inputted to the laser crystal 8, is under excitation input limitation at a breakdown threshold value of the crystal, and it is difficult to have high output.
In order to raise excitation efficiency, the fundamental wave with the wavelength λ1 is needed to be on the same optical axis as the fundamental wave with the wavelength λ2. Because the concave mirror 12, the second plane reflection mirror 15 and the third plane reflection mirror 16 are arranged on a common optical axis, it is difficult to perfectly match the two optical axes with the wavelengths of λ1 and λ2 by adjusting the concave mirror 12, the second plane reflection mirror 15 and the third plane reflection mirror 16.
Further, the nonlinear optical medium 9 must be arranged on a portion of the laser beam with higher energy density (beam waist) in order to have higher efficiency. The beam waist (ω) is obtained from the equation (1) given below, and the position is different if the wavelength λ is different. Therefore, as shown in FIG. 6, if the laser crystal 8 is provided commonly for the wavelengths λ1 and λ2, the nonlinear optical medium 9 cannot be placed at the position of the beam waist of the wavelength λ1 and of the beam waist of the wavelength λ2, and conversion efficiency is decreased.                     ω        =                              {                          λ              ⁢                                                                    [                                          L                      ⁡                                              (                                                  R                          -                          L                                                )                                                              ]                                                  /                π                                      }                                              (        1        )            where L denotes length of the resonator, provided neglecting, for symplification, crystal effects (thermal lens effects, optical path change, etc.), and R is curvature of the concave mirror.